A grating monochromator is an optical device which disperses the light from a radiant source into its constituent wavelengths and focuses one wavelength at the exit slit of the device, there to be observed or utilized.
Monochromators usually comprise a housing with an entrance slit to receive light into the housing, a grating positioned within the housing to receive and disperse the light passing through the entrance slit, and an exit slit in the housing positioned to transmit a narrow bandwidth of the light dispersed by the grating. The grating can pivot within the housing to direct different wavelengths of the spectrum through the exit slit for use or observation.
A grating spectrograph is an optical device related closely to a monochromator. Like a monochromator, a spectrograph disperses a beam of light into its constituent wavelengths. However, a monochromator focuses only a narrow band of wavelengths at the exit aperture or slit, while the design of the spectrograph is such that the entire spectrum of wavelengths present in the source is focused at the exit focal plane. Thus, a spectrograph is a polychromator, as multiple wavelengths are present simultaneously in linear array at the exit focal plane. The position along the spectrum determines which wavelength is present.
Many monochromators and spectrographs function with a single grating. A double grating system, however, offers improved spectral resolution and greatly reduced stray light ("stray light" being light of a wavelength other than that intended).
The well-known equation (ref 1) describing the behavior of a plane, reflective, diffraction grating is shown below. If i is the angle of incidence, d the angle of diffraction, s the distance between the rulings, n the order of the spectrum, then the wavelength (lambda) is EQU LAMBDA=s/n(sin i+sin d) (1)
If only first order diffraction is considered, n=1, s=constant for any particular grating, therefore, EQU LAMBDA=constant (sin i+sin d) (2)
Examination of equation 2 reveals that there are three ways that the wavelength, lambda, can be changed or scanned.
Case I: d is fixed and i is varied. This is the case of a device with a fixed grating, fixed exit slit, and moving entrance slit. PA0 Case II: i is fixed and d is varied. This is the case of a device with a fixed grating, fixed entrance slit, and moving exit slit. PA0 Case III: Both i and d are varied. This is the case of a device with a rotatable grating and fixed entrance and exit slits.
The present invention corresponds to Case I. That is, the single grating version uses a moving entrance slit so that the angle of incidence i varies as the slit moves. The double grating version can be viewed in the same way, since the moving intermediate slit is the entrance slit to the second half of the double system.
In the single grating version of the present invention, the entrance slit moves laterally along the length of a linear light source, so that the light admitted to the device strikes the diffraction grating at varying angles of incidence. Since the exit slit of the monochromator is fixed in position, only one wavelength of light will emerge from it since only one wavelength of light satisfies the grating equation for each different angle of incidence and fixed angle of diffraction.
The double grating version of the present invention is a form of subtractive double monochromator. These devices have been described and analyzed in the literature (see references 1, 2, 3, 4, and 5).
Double monochromators are devices which use two monochromators where the exit slit of the first monochromator is the entrance slit of the second monochromator. Thus, a double monochromator has an entrance slit, an intermediate slit, and an exit slit and the first dispersing element (diffraction grating) forms a real spectral image in the plane of the intermediate slit. If the dispersing elements of the monochromators are arranged so that the dispersion of the second element adds to the dispersion of the first element, the device is termed an "additive double monochromator." These devices exhibit low stray light and high dispersion and are very widely used. If the dispersing elements are arranged so that the dispersion of the second element subtracts from the dispersion of the first, the device is termed a "subtractive double monochromator." Subtractive double monochromators also provide very low stray light since the second half of the device reduces the already low stray light passing from the first half; this property is particularly important in spectrophotometric applications requiring high photometric accuracy.
The double grating version of the present invention can be described as a subtractive double grating monochromator in which the intermediate slit is rapidly and repeatedly moved (in a way to be described) across the real spectral image formed in the plane of the intermediate slit, thereby causing a rapid sweeping or scanning of the wavelength of the light admitted to the second half of the device wherein the light is de-dispersed or combined to a single beam irrespective of wavelength which beam emerges from the exit aperture of the monochromator.